Conventionally, as a gold wire to connect, in a semiconductor device, an IC chip electrode to an externally drawn-out lead, a gold wire having a purity of 99.99 mass % (hereinafter simply referred to as ‘%’ in place of “mass %”) or higher made of a high purity gold doped with a trace quantity of another metallic element has been used widely for the reason of high dependability. The usage of such a pure gold wire as this is such that one end of the wire is connected to a pure aluminum (Al) pad or an aluminum alloy pad on an IC chip electrode by means of a ultrasonic wave-assisted thermal bonding method, and the other end is connected to an external lead or the like on a circuit board, and thereafter the circuit is sealed with a resin to make a semiconductor device. Such a pure aluminum (Al) pad and an aluminum alloy pad as these are usually formed by vacuum deposition.
As another Ag—Au—Pd ternary alloy bonding wire, there is disclosed one in Japanese Patent Application Publication H09-272931 “a gold alloy fine wire for semiconductor devices characterized by consisting of, by weight, 10-60% Ag and 0.005-0.8% Mn, plus in combination with the above, 0.005-5% by weight, in total, of at least one element selected from Cu, Pd and Pt, and 0.0002-0.03% by weight, in total, of at least one element selected from Ca, Be, La, Ce and Y, and the balance being gold and inevitable impurities.”
Also, Japanese Patent Application Publication 2000-150562 discloses “a gold alloy fine wire for bonding with semiconductor device made of a gold alloy containing Ag by 1 to 50 wt. %, Pd by 0.8 to 5 wt. %, Ti by 0.1 to 2 wt. ppm, plus one or more selected from Ca, Be and La by 1 to 50 wt. ppm and the remaining part consisting of gold and inevitable impurities.” The bonding wires such as these were developed as alternative wires for gold wire and were intended to reduce the consumption of costly gold (Au) as well as to enable formation of melt ball in the atmosphere.
However, in the case of a bonding wire made of an Ag—Au—Pd alloy with the silver (Ag) content exceeding 60%, it has been experienced that when it is ball-bonded in the natural atmosphere, some oxides separate in the surface of the melt ball with a result that the ball does not bond well to the target such as an aluminum alloy pad. Such oxides originate from an oxidizing non-noble metal element or inevitable impurities contained in the raw Ag, and as such elements added in trace amounts and the inevitable impurities separate in the surface of the melt ball, they couple with the oxygen in the atmosphere and turn oxides.
When the content of silver (Ag) exceeds 60%, the rigidity of the bonding wire itself becomes so increased that it was a common practice to subject the molded bonding wire to a final annealing. For example, in Japanese Patent Application Publication H03-74851 (hereinafter referred to as “IP Publication 1”) there are disclosed “wires made of Ag—Pd alloy, Au—Ag—Pd alloy, etc. which have been heat-treated at temperatures higher than the respective recrystallization temperature,” and Japanese Patent Application Publication 2010-171378 discloses a method for manufacturing a wire made of a gold-silver-palladium alloy which has been drawn to a wire diameter of 0.050 mm-0.010 mm and contains gold by 8.00-30.00 wt. % and silver by 66.00-90.00 wt. % and palladium by 0.01-6.00 wt. %, consisting of cleansing and drying the wire surface and then annealing.
However, in the case of a bonding wire made of an Ag—Au—Pd ternary alloy, a fact exists that the greater the content of silver (Ag) is the more oxygen is taken in from the atmosphere so that the surface tension becomes so weak that the shape of the melt ball gets unstable and also what with the above-mentioned separation of oxides it was only possible to achieve a connection by means of ultrasonic wedge bonding.
As the semiconductor devices sealed with resin began to be used in IC's for automotive vehicles wherein high reliability is demanded in spite of the severe circumstances of heightened temperatures and also in IC's for high frequency waves and high brightness LED in which the operational temperature rises quite high, the bonding wires made of Ag—Au—Pd ternary alloys which have been finished with annealing are not capable of maintaining their properties in such high temperature situations. For these reasons the fact is that the bonding wires of Ag—Au—Pd ternary alloys have not yet been put to use.